1. Field of the Invention
The present invention relates to a discharged-sheet monitoring mechanism and method for a printer such as an inkjet printer.
2. Description of the Related Art
In recent years, image forming apparatuses which perform large-volume printing and copying have been heavily used. Such image forming apparatuses include printers, facsimiles, copiers and the like. In general, an image forming apparatus is equipped as standard with a sheet discharge device configured to stack sheets having images formed thereon on a paper receiving tray. Such a sheet discharge device is desired to have a sheet stacking unit having a huge capacity. However, the stacking capacity of the sheet discharge device has limitations. When large-volume printing causes sheets to be discharged beyond the stacking capacity of the sheet discharge device, a discharged sheet jam, such as a paper jam or a fall of a sheet from the stack, occurs.
To prevent an overflow (full state) which is a cause of such a discharged sheet jam, there are techniques disclosed in Japanese Patent Application Publications Nos. 2005-239323 and Hei 5-301430. The technique disclosed in Japanese Patent Application Publication No. 2005-239323 includes: determining whether or not the amount of sheets stacked on a paper receiving tray has reached a maximum amount allowed; and then stopping units upstream of the paper receiving tray if necessary. Here, the determination is made by use of a method in which a detection sensor detects a sheet stack height on the paper receiving tray or a method in which a further printable amount is detected by counting the number of printed sheets. In the technique disclosed in Japanese Patent Application Publication No. Hei 5-301430, a remaining discharged-sheet stacking amount of a sheet discharge device is detected, and a notification such as a warning or an error display is provided to a user to prompt the user to take away printed sheets discharged to the sheet discharge device.
However, in the further printable amount detection method disclosed in the above-described Japanese Patent Application Publication No. 2005-239323, consideration is not given to the different thicknesses of various types of print sheets. Accordingly, this technique does not enable a determination on an accurate amount of sheets further printable until the amount of sheets discharged to the paper receiving tray reaches the maximum amount allowed. In the case of Japanese Patent Application Publication No. Hei 5-301430 in which a notification is provided to a user, only a notification is provided when the amount of sheets discharged to the paper receiving tray reaches a certain level, but the amount of sheets further dischargeable cannot be displayed. Accordingly, users cannot make the best use of a maximum amount allowed of the paper receiving tray, and spend a lot of time on printing work and sheet-discharging work.
Further, there is a case where large-volume printing so-called JOB data is executed as a single process. In this case, when an overflow occurs on the paper receiving tray during job processing, the entire job processing is stopped in the techniques disclosed in the above-described patent documents. Accordingly, despite a single batch of job data, print sheets discharged to the paper receiving tray are separated into two or more groups due to the stoppage of job processing. This may considerably reduce the efficiency of user's work.